1. CEPC Partial Double Ring Parameter Update
Dou Wang, Jiyuan Zhai, Feng Su, Yuan Zhang, Yiwei Wang, Bai Sha, Huiping Geng, Tianjian Bian,

2. CEPC Partial Double Ring Layout
SU Feng
IP1_ee
IP3_ee
IP2_pp
IP4_pp
3Km RF
1/2RF
IP1_ee/IP3_ee, 3Km
IP2_pp/IP4_pp, m
4Straights, 849.6m
4Long ARC, 120*FODO, m
4Short ARC, 100*FODO, m
C= m
Advantage:
Avoid pretzel orbit
Accommodate more bunches at Z/W energy
Reduce AC power with crab waist collision

3. Machine constraints / given parameters
Energy E0
Circumference C0
NIP
Beam power P0
y*
R=y/x
Emittance coupling factor 
Bending radius 
Piwinski angle 
luminosity enhancement by crab waist Fl ~1.5
Phase advance per cell (FODO)

4. Parameter choice – step 1
Beam-beam limit:
Fl: y enhancement by crab waist
*J. Gao, emittance growth and beam lifetime limitations due to beam-beam effects in e+e- storage rings, Nucl. Instr. and methods A533（2004）p

5. Parameter choice – step 2

6. Parameter choice – step 3
BS life time: 30 min
y:
-- phase advance/cell, -- bending angle/cell.
Estimate :

7. Parameter choice – step 4

8. Parameter choice – step 5
Effective bunch length: overlap area of colliding bunches
Hour glass effect:

9. Parameter choice – step 6
Vrf , s
Energy acceptance from RF:

10. Parameter choice – step 7
Loss factor
HOM power per cavity

11. Primary parameter design
Pre-CDR
high lumi.
low power Z
Number of IPs 2
Energy (GeV)
120
45.5
Circumference (km) 54
SR loss/turn (GeV)
3.1
2.96
0.062
Half crossing angle (mrad)
14.5
5.2
11.5
4.3
16.5
Piwinski angle
2.6
Ne/bunch (1011)
3.79
1.2
2.81
1.0
0.37
Bunch number 50
158 40
114
1100
Beam current (mA)
16.6
16.9
10.1
36.2
SR power /beam (MW)
51.7 30
2.2
Bending radius (km)
6.1
6.2
Momentum compaction (10-5)
3.4
3.0
2.8
2.5
5.4
IP x/y (m)
0.8/0.0012
0.306/0.0012
0.036/0.0012
0.22/0.001
0.03/0.001
0.3/0.001
Emittance x/y (nm)
6.12/0.018
3.34/0.01
3.07/0.0093
2.67/0.008
2.56/0.0078
1.18/0.0069
Transverse IP (um)
69.97/0.15
32/0.11
10.5/0.1
24.3/0.09
8.8/0.088
18.8/0.083
x/IP
0.118
0.04
0.015
0.02
y/IP
0.083
0.11
0.042
VRF (GV)
6.87
3.7
3.9
3.6
0.28
f RF (MHz)
650
Nature z (mm)
2.14
3.3
3.2
Total z (mm)
2.65
4.4
4.0
4.2
HOM power/cavity (kw)
0.95
1.5
0.47
0.73
Energy spread (%)
0.13
0.05
Energy acceptance (%)
Energy acceptance by RF (%) 6
2.4
2.0 n
0.23
0.49
0.46
0.08
Life time due to beamstrahlung_cal (minute) 47 53 32 41
F (hour glass)
0.68
0.75
0.69
0.7
0.67
Lmax/IP (1034cm-2s-1)
2.04
2.97
3.07
2.03
2.08
1.01

12. Arc redesign-lower emittance
Length of FODO cell: 47.2m
Phase advance of FODO cells: 60/60 degrees
Emittance: 7.9nm, p=5.38E-5
Bunch length: 2.3mm
Length of FODO cell: 37.5m
Phase advance of FODO cells: 60/60 degrees
Emittance: 4.3nm , p=2.25E-5
Bunch length: 3.3mm

13. Arc redesign-ultra low emittance
Length of FODO cell: 37.5m
Phase advance of FODO cells: 90/60 degrees
Emittance: 2.3nm, p=1.07E-5
Bunch length: 3.3mm
Dispersion supressor:
Angle(BDIS1)= E-03
Angle(BDIS2)= E-04
Angle(B0)= E-03
BDIS1
BDIS2 B0 B0

14. summary
CEPC AP group and RF group has collaborated to give a more realizable parameter considering cavity HOM power.
Based on crab waist scheme, we get a set of Z parameter with 1.0*1034cm-2s-1 luminosity using 1100 bunch.
Higgs’s parameters have been updated to reduce the cavity HOM power under 1kw. Further improvement is undergoing to make same crossing angle as Z parameter.